US President Donald Trump appeared to confirm on Friday that he was under investigation and seemingly took aim at a senior Justice Department official, in a tweet that seemed to encapsulate his frustration with an ongoing probe into alleged Russian meddling in the 2016 presidential election.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," the president wrote in his tweet.

The tweet came after reports in The Washington Post on Wednesday claiming that Trump was being investigated for potential obstruction of justice by Rob Mueller, the special counsel investigating alleged Russian involvement in last year's vote and possible collusion with the Trump campaign.

I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt

A US official, speaking to Al Jazeera on condition of anonymity, denied that Trump was confirming the investigation in his tweet. The official claimed Trump was referencing the Washington Post's claims, which he said were based on "anonymous" and "illegal" leaks.

Trump's tweet that he is being investigated "by the man who told me to fire the FBI Director" may refer to Deputy Attorney General Rod Rosenstein.

Rosenstein, in a memo to Trump, had raised concerns over FBI Director James Comey's performance.

While the Trump administration initially said that letter was the reason the president fired Comey on May 9, Trump later said he did so because of the "Russia thing".

And it was Rosenstein who appointed Mueller as special counsel to the Russian investigation.

Also unclear was whether the president's comment apparently confirming he was under investigation was based on direct knowledge or media reports that Mueller is examining whether Trump obstructed justice by firing Comey.

Al Jazeera's Kimberly Halkett, reporting from Washington, DC, said the tweet has caused confusion about to whom Trump is referring, because Mueller was not special counsel when Comey was fired and could not have advised him to take that action.

Trump has also said that decided to fire Comey himself, and Rosenstein's memo followed that decision.

"What does make sense though is this seems to be an indication the president is growing increasingly frustrated with the ongoing reporting of this case and the investigation that has broadened in scope, and is lashing out on social media as a result," she said.

Increasing pressure

In another tweet earlier on Friday, Trump wrote:

After 7 months of investigations & committee hearings about my "collusion with the Russians," nobody has been able to show any proof. Sad!

Trump's tweets come after the top lawyer for his transition team warned team officials to preserve all records and other materials related to the Russia probe. An official of Trump's transition confirmed the lawyer's internal order sent on Thursday.

The transition official spoke to the Associated Press news agency on condition of anonymity because the person was not authorised to discuss post-election decisions publicly.

The transition order came the same day that Vice President Mike Pence confirmed that he had hired a private lawyer to represent his interests in the expanding probe. Pence headed the Trump transition until inauguration day.

The order casts a wide net on documents that could shed light on ties between Trump's presidential campaign and representatives of Russia's government.

In written testimony for the Senate Intelligence Committee earlier this month, Comey claimed Trump told him at a meeting in the White House in February: "I hope you can see your way clear to letting this go, to letting Flynn go." Trump has denied any wrongdoing.

The memo sent on Thursday asks for records related to foreign travel, contacts with Russian "officials, agents or nationals" and background investigations into the top Trump associates now targeted by Mueller's probe.

The memo asks for the preservation of electronic communications and data, telephone logs, audio recordings, videos, calendars and other items.

The transition official said the organisation has also separately asked the General Services Administration to preserve records from the Trump transition that were transferred to its facilities after Trump's inauguration.

The transition, a non-profit structurally separate from the Trump campaign, continues to operate with a small staff.